Was this a professionally-rebuilt generator or a used one?
GM has had a very common repeat failure problem since they redesigned their generators for the 1987 model year. They went from what I consider the world's second-best design to by far the worst. Specifically, due to the switching circuitry used by the voltage regulator, these generators develop huge voltage spikes that can damage the internal diodes and regulator, and interfere with computer sensor signals. The battery is the key component in damping and absorbing those harmful spikes, but as they age and the lead flakes off the plates, they lose their ability to do that. This is when we start to see multiple generator failures. It is not uncommon to go through four to six replacement generators in the life of the vehicle.
The secret to reducing this high number of repeat failures is to always replace the battery at the same time when you have to replace the generator, unless that battery is less than about two years old. The old battery will work just fine in an '86 or older car.
Given the intermittent nature of the problem, if this was a used generator you installed, the best suspect is worn brushes. Those can be replaced rather easily on most other generators, but it is just about impossible to do that on GM's units. It is a lesson in frustration just getting them apart without breaking the thin tabs off the diode block, and once you do get it apart, there is no way to test the voltage regulator or the brushes. Even if you had an electronic tester for the voltage regulator, we know the generator works at times, and this could be while you're testing the regulator. That won't tell you if it is what's failing intermittently.
Since you can't test any of the important internal components, you'll want to replace them all to be on the safe side. Given the cost of those parts, and the time involved, it is a better value to buy a rebuilt generator from an auto parts store. Be aware that you should check prices and warranties at multiple stores. My friend is on his fourth replacement generator for his '99 Suburban. He found one some years ago for $250.00 with a one-year warranty, and at a different store, he found one for $50.00 less with a lifetime warranty.
The 14.6 volts you found earlier is perfect, right in the high end of the acceptable range of 13.75 to 14.75 volts. Check that again when the problem is occurring. If the battery voltage is still within that range, the voltage regulator is defective. It is turning the "Battery" warning light on when it shouldn't be.
If you find battery voltage has dropped to around 12.6 or less, check the voltages on the back of the generator. The large bolted-on wire must have the same voltage as you found at the battery. If it is significantly different, suspect the large bolted-in fuse has a loose connection.
Next, check the voltage on the black / white wire pointed out in this diagram. It must also have full battery voltage on it. Fuses are not intermittent, but they can be loose in their sockets. That could cause an intermittent problem, but a better suspect would be loose or corroded mating terminals in the connector shown in that circuit.
If those voltage on the wires are okay while the problem is occurring, the generator is the logical suspect.
For the benefit of others researching this topic, here's a link to a related article:
https://www.2carpros.com/articles/how-to-check-a-car-alternator
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Monday, February 24th, 2020 AT 2:57 PM
